Link Access Procedure, Balanced (LAPB) implements the data link layer as defined in the X.25 protocol suite. LAPB is a bit-oriented protocol derived from HDLC that ensures that frames are error free and in the right sequence. LAPB is specified in ITU-T Recommendation X.25 and ISO/IEC 7776. It can be used as a Data Link Layer protocol implementing the connection-mode data link service in the OSI Reference Model as defined by ITU-T Recommendation X.222.
LAPB is used to manage communication and packet framing between data terminal equipment (DTE) and the data circuit-terminating equipment (DCE) devices in the X.25 protocol stack. LAPB is essentially HDLC in Asynchronous Balanced Mode (ABM). LAPB sessions can be established by either the DTE or DCE. The station initiating the call is determined to be the primary, and the responding station is the secondary.